This invention relates to an improved methanation catalyst and method for producing methane from carbon monoxide and hydrogen. The catalyst is termed a methanation catalyst because the major component of the reaction product, on a moisture-free basis, is methane. More specifically, this invention relates to a methanation catalyst and its use in a method of converting a gaseous feed containing the reactants hydrogen and carbon monoxide present in a molar ratio of about three to one, generally in the presence of methane and minor amounts of other gases such as nitrogen, carbon dioxide, steam, and the like, into a gaseous product stream containing a major proportion of methane, disregarding water. The catalyst is formed by reducing a complex of the combined oxides of nickel, copper and molybdenum supported on a catalyst support, the relative ratios of nickel, copper and molybdenum being defined within relatively narrow limits for preferred operation.
The prior art is replete with numerous metallic catalysts which have been utilized, both in supported and non-supported form, to catalyze the reaction of carbon monoxide and hydrogen. In general, though some methane is formed with prior art metallic catalysts, most are directed to the standard Fischer-Tropsch reaction for the synthesis of hydrocarbons starting with carbon monoxide and hydrogen, and therefore are specifically directed to the formation of hydrocarbons of relatively high molecular weight.
Though the methanation of carbon monoxide has been referred to in numerous references, including those which teach the Fischer-Tropsch synthesis, it is only in the face of declining gas reserves that a great deal of attention has been focused on arriving at a practical and economical process for methanation of feed streams containing high CO content, and of course, a catalyst which will fulfill the demanding requirements of such a process. The large number of catalytic elements disclosed in the prior art are of little help with respect to obtaining a commercially significant catalyst to upgrade less accessible energy sources such as coal to methane, which catalyst will help stem the shortage of natural gas as a vital energy source. It will be apparent from disclosures of prior art Fischer-Tropsch catalysts that the formation of methane in any substantial amount is regarded as detrimental to the catalyst's performance. Preferred catalysts are those which yield higher hydrocarbons, preferably containing in excess of three carbon atoms. Disclosures with respect to the Fischer-Tropsch synthesis catalyst are so all-encompassing as to cover combinations of almost any metallic element in the periodic table. The disclosures of Fischer-Tropsch catalysts which might suggest the reduced oxides of nickel, copper and molybdenum equally suggest innumerable other combinations as essential catalytic ingredients. Moreover, the proportions of components specifically suggested for prior art commercial methanation catalysts are found to be generally unsuited for commercial methanation at elevated pressures in excess of about 500 psi, and where carbon monoxide is present in excess of about 10 percent of the feed, because of their instability under intense exothermic heats of reaction generated under those conditions.
Several processes are currently being developed for coal gasification. All the processes require final methanation of a mixture including H.sub.2 and CO to yield a pipeline quality product.
The catalyst of the instant invention, and the method for using it, are especially directed to large scale industrial operations where large amounts of carbon monoxide and hydrogen, at a pressure in excess of 300 psig., are to be continuously and reliably converted primarily to methane and water. More specifically, the instant catalyst and the method for using it are directed to the conversion of gaseous products obtained by the gasification of coal, or from offgases from the retorting of oil shale or the liquefaction of coal, or the gasification of heavy petroleum residues, and the like, all of which are characterized by producing carbon monoxide-rich gas, usually in the presence of large quantities of hydrogen. The gases so produced have a low heating value, that is, less than 500 Btu. per standard cubic foot of gas, and contain minor amounts of other gases, particularly methane, carbon dioxide, nitrogen oxides, and the like. Since the methanation reaction is strongly exothermic it is essential that the catalyst be thermally stable. Also, since efficient recovery of the heat generated in the methanation reaction is a significant factor in the overall efficiency of the process it is desirable to carry out the methanation reaction at as high a temperature as possible.
Since it is necessary to supply pipeline gas at high pressure it is desirable to conduct a methanation reaction at elevated pressure in the range from about 300 psig. to about 1500 psig. Though high pressure reaction conditions benefit both the rate and equilibrium of the methanation reaction, the effect on the methanation catalyst is to increase the severity of the methanation reaction and to subject the catalyst to a high heat release in the reaction zone.
Numerous references which teach methanation with nickel catalysts illustrate the problems set forth hereinabove. The problem of thermal degradation of the catalyst has been attacked by process and equipment design modifications which tend to increase cost and decrease efficiency of the process. For example, several well-regarded processes use a recycle gas to dilute the feed to the methanator so as to maintain the CO concentration below 10% and usually below 5%. Another well-regarded process utilizes heat-transfer surfaces coated with catalyst in order to control the exothermic heat.
Prior art synthesis catalysts, for instance Fischer-Tropsch catalysts, are deliberately selected and the processes are operated to minimize the formation of CH.sub.4 and to maximize the yield of higher molecular weight products. Other synthesis catalysts are disclosed to contain the transition elements of groups VB, VIB and VIII promoted with still other elements, thus teaching that almost any combination of elements in a large number of Groups will provide an effective synthesis catalyst. With respect to methanation catalysts it is stated:
The Group VIII transition elements Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, and Pt, particularly the former three, have been found to be effective methanation catalysts.
The problem areas in catalytic methanation are generally agreed to be associated with the strong exothermicity of the reaction which can cause excessive temperature and catalyst deactivation by sintering, carbon deposition and sulfur poisoning of the catalyst. (See "Catalysts for Coal Conversion", by John L. Cox, Symposium on Clean Fuels from Coal, IGT, Chicago (September 1973) pp. 311-340.
We know of nothing in the prior art which teaches that the supported combined oxides of nickel, copper, and molybdenum, activated by reduction on the support, will provide an effective methanation catalyst characterized by an insensitivity to high concentrations of carbon monoxide at pressures in excess of 500 psi, thermal stability at an operating temperature in the range from about 400.degree.C to about 625.degree.C, and a satisfactory conversion of carbon monoxide.
It should be noted that methanation reactions have been employed in the clean-up of reformer product gases prior to their introduction into fuel cells, and for the removal of carbon monoxide from the feed to reactors in ammonia synthesis plants. Most such methanation catalysts are primarily nickel-based and include no copper, are not suitable for converting CO which is present in excess of about ten percent, are easily poisoned or rendered inactive, and therefore have no utility in the particular methanation process of this invention. This is acknowledged in the statement:
Catalytic methanation has been known for 70 years and has been utilized extensively in removing small amounts of CO from hydrogen-containing gases . . . The composition of a typical catalyst is 77% Ni oxide and 22% Al.sub.2 O.sub.3. It is easily poisoned by sulfur and also rendered inactive by carbon deposition and by sintering.
A variety of reaction systems has been used to overcome the severe problems of the high heat release. (See "Future Catalytic Requirements for Synthetic Energy Fuels", by G. Alex Mills, ACS Meeting, Boston (April 1972); Div. of Fuel Chemistry Preprints, Vol. 16, No. 2, pp. 107-123.
It should also be noted that prior art synthesis catalysts generally utilize the product of a reforming operation in which methane was first converted to carbon monoxide and hydrogen. The carbon monoxide and hydrogen is then utilized to synthesize higher molecular weight products, and not methane. A typical reference in this art is U.S. Pat. No. 2,500,516 to Carpenter, wherein it is disclosed that a catalyst for the synthesis step may be metallic iron, cobalt, or nickel, either alone or on a suitable carrier such as kieselguhr, silica gel, alumina, etc., and including one or more promoter oxides such as the oxides of magnesium, chromium, manganese, aluminum, copper, etc. (Column 6, lines 50-55).
It has long been known that nickel is a highly active methanation catalyst; and also, that molybdenum is a methanation catalyst which has excellent thermal stability and long life, but substantially lower activity than nickel, and poorer selectivity for methane. It is also known that the reduced combined oxides of nickel and molybdenum are initially active, but rapidly obtain the characteristics of reduced molybdenum oxides alone. Reduced molybdenum oxides, with or without a support, are insufficiently selective for the production of methane and are therefore of minimal interest. It is further known that nickel containing a small amount of copper initially provides high conversions of carbon dioxide to methane but a concentration of copper approaching four percent of combined nickel and copper rapidly erodes the activity of the catalyst (see "Nickel, etc., Catalysts for the Hydrogenation of Carbon Dioxide", by L. E. Cratty, Jr., and W. W. Russell, Journal of Am. Chem. Soc., Feb. 20, 1958, Vol. 80, p. 767). Since it is generally accepted that the behavior of a catalyst in the methanation of carbon dioxide is indicative of its behavior in the methanation of carbon monoxide, it is quite unexpected that when copper in excess of four percent is combined with nickel and molybdenum, a thermally stable, active methanation catalyst would result which is effective even at ratios of hydrogen to carbon monoxide which are substantially lower than 3 and as low as 1.
The thermal stability, high activity and ratio flexiblity of our catalyst are also unexpected in light of the recent observations confirming the finding of Sabatier and Senderens that ". . . cobalt also promoted the reaction but that copper, iron, platinum and palladium did not form active catalysts," and further stating that "Thus, by 1925 all of the metals now considered active for methanation of carbon oxides had been identified. In terms of metals important for methanation, the list could now be shortened to Ru, Ni, Co, Fe and Mo." (see "Catalytic Methanation" by G. A. Mills and F. W. Steffgen in Catalysis Reviews, Vol. 8(2), pg. 159-210, 1973).
Also known is a gas equilibration catalyst prepared by co-precipitating nickel, aluminum, copper or zinc and chromium as hydroxides, carbonates or basic carbonates, which on calcination in the presence of oxygen or air, form mixed oxides. The co-precipitated metal hydroxides are impregnated with a barium salt that is decomposed to barium oxide by the calcination. Such a catalyst is disclosed in U.S. Pat. No. 3,444,099 to Taylor et al. as being effective to convert exhaust gases from automobiles at a temperature of 485.degree.C and a space velocity of 10,000 volumes of gas at S.T.P. per volume of catalyst per hour. This catalyst includes barium oxide or a metal oxides type promoter such as potassium, cesium, strontium, and the like. Specifically, the reference includes examples of a catalyst containing nickel, molybdenum, copper, chromium, aluminum and barium in which chromium oxide and barium oxide are necessary components. The catalysts (I and K in Table I) convert relatively low percentages of carbon monoxide after short periods of operation. Moreover, there is no reason to conclude that the catalysts which are suitable for low conversion to methane at low CO concentration might also be suitable, with certain modifications, for high conversion to methane at relatively high CO concentration. In particular, there is no suggestion in the art as to how these catalysts may be modified to provide a per pass conversion in excess of 80 percent of carbon monoxide present in a feed containing in excess of 5% CO. Note also, that the disclosure is for the use of the catalyst in the presence of large quantities of normal butane and butylenes.
From the foregoing it will be apparent that much effort has been expended on the development of an effective gas equilibration catalyst wherein the product contains a major amount of methane. Specifically, applicants know of no gas equilibration catalyst for carbon monoxide present in excess of 5% in a substantially olefin-free feed, having the specific combination of the reduced oxides of nickel, copper, and molybdenum as its essential catalytic ingredients on a suitable support, to the substantial exclusion of all other catalytic ingredients. The catalyst of this invention and the method of its use provides a practical and economical process which profers a solution to the burgeoning problem of a dwindling supply of natural pipeline gas, a profitable utilization of industrial off-gases containing large amounts of carbon monoxide and hydrogen, and specifically, a commercially viable scheme for utilizing the gasification of coal.